1. Field of the Invention
The present invention relates to a process for inhibiting the polymerization of (meth)acrylic acid, (meth)acrylic esters, and other easily-polymerizable compounds in distillation of these compounds.
2. Description of the Related Art
Distillation is a procedure for separating components of a liquid mixture comprising two or more liquids having different boiling points. In distillation, the feedstock liquid is heated and vaporized and a vapor mainly containing a low boiling component is condensed. When the components have largely different relative volatilities, they can be easily separated. However, vaporization and condensation of only a portion of the liquid mixture cannot separate the components in expected concentrations in ordinary cases. Distillation is therefore generally performed by refluxing a portion of a condensate from the top of the column to the surface of a tray or packing in the column to ensure a vapor rising in the distillation column to thereby appropriately come in contact with a liquid flowing downward from the top, and distilling off a low boiling component from the top and withdrawing a high boiling component from the bottom of the column. Such a distillation unit for use in distillation generally includes a distillation column, a thin film evaporator, and a reactor installed a column. There may be further provided a condenser, a reboiler, and/or an external heater. The thin film evaporator may further be provided with a column on a top.
FIG. 1 is an example of a cross sectional view illustrating a plate distillation column 1. Such a plate distillation column is generally used in distillation of (meth)acrylic acid and (meth)acrylic esters (hereinafter briefly referred to as xe2x80x9c(meth)acrylic acid or its esterxe2x80x9d). Distillation using this distillation column will now be described by taking, as an example, the use of a feedstock liquid containing the (meth)acrylic acid or its ester as a main component and acetic acid as an impurity. The feedstock liquid is supplied from a column wall into the distillation column (L1). Acetic acid, a low boiling component, is vaporized by heating and rises in the column, is introduced into a condenser 2 (L2) from the top of the column, is condensed, and is taken out of the system (L3). Separately, a high boiling component such as the (meth)acrylic acid or its ester is fed to a subsequent step from the bottom of the column (L4) as a refined liquid, and a portion of the refined liquid is returned via a reboiler 3 to the distillation column (L5). The condensate contains not only the impurity acetic acid but also the (meth)acrylic acid or its ester, and a portion of the condensate is returned as a reflux liquid to the column (L6), and is again subjected to gas-liquid contact in the distillation column to allow the (meth)acrylic acid or its ester to flow down to the bottom of the column.
FIG. 2 is an illustrative view of a distillation unit for use in a reaction of a (meth)acrylic ester. As reactants, (meth)acrylic acid and a liquid containing an alcohol are fed (L18)to a reactor 18 using an ion exchange resin, are agitated with an agitator 21 and are put (L9) into a jacket of the reactor with steam for reaction, and formed water is expelled from the reactor as a vapor by distillation(L17). A product liquid containing an esterified (meth)acrylic ester is withdrawn from an outlet (L15) and is supplied into a succeeding purification step (L16). The vapor from the reactor 18 is fed (L17) to a distillation column installed on the top of the reactor 18 and is subjected to gas-liquid contact while supplying a liquid (L10, L11) from the top of the column. The liquid components such as the (meth)acrylic ester, (meth)acrylic acid, and alcohol fall into the reactor (L18). A vapor distillated from the top of the column is introduced into a condenser 24 (L12), is condensed, and is fed into a decanter 23 (L13). After separation, a separated water phase is eliminated (L14) and a separated oil phase is refluxed (L11) to the distillation column 17. If necessary, the reactant alcohol is fed (L10) from the top of the column.
The (meth)acrylic acid or its ester is highly easily polymerizable compounds, and a polymerization inhibitor such as hydroquinone or phenothiazine is generally added to the feedstock liquid mixture and/or the reflux liquid. To inhibit the polymerization, a technique of supplying a gas containing molecular oxygen from a lower part (L7) of the distillation unit is also employed.
Japanese Examined Patent Publication No. 63-11921 discloses another distillation technique. Specifically external heaters, e.g., heating jackets, are provided on an evaporator above and below an evaporation surface. These external heaters inhibit vapor from condensing and polymerizing on an inner surface of a gas section of the evaporator without any polymerization inhibitor.
By these conventional techniques, however, the polymerization cannot be always inhibited, and the operation of the distillation unit is relatively frequently stopped to remove polymer, because the formation of such polymer deteriorates the purification efficiency.
The present invention has been accomplished under these circumstances, and an object of the invention is to provide a process which is capable of inhibiting the polymerization of easily polymerizable compounds such as (meth)acrylic acid and (meth)acrylic esters in a distillation unit during the distillation of these compounds.
Specifically, the invention provides a process for inhibiting the polymerization of a easily polymerizable compound in distillation of the compound using a distillation unit. In the process, a liquid is supplied to a constitutive member by a spraying and supplying means, the constitutive member is placed in the distillation unit, and the liquid has substantially the same composition with that of a liquid surrounding the constitutive member. The temperature of the liquid supplied by the spraying and supplying means may be preferably equal to or lower than that of liquid surrounding the constitutive member in the distillation unit. Further, the liquid may preferably have a wetting liquid rate (of the sprayed and supplied liquid, a volume of a liquid attaching to and wetting the constitutive member) of 0.5 m3/m2 hr or more relative to the surface area of the constitutive member. The liquid supplied by the spraying and supplying means preferably contains a polymerization inhibitor. It is also preferred to supply a gas containing molecular oxygen to the distillation unit. Oxygen gas and air may be used as the molecular-oxygen-containing gas. The proportion of the molecular-oxygen-containing gas (when the gas contains another component in addition to oxygen gas, the proportion in terms of oxygen gas) is preferably about 0.01 to 5% by volume, and more preferably about 0.02 to 3% by volume relative to the evaporation volume rate (at standard temperature and pressure) of the easily polymerizable compound in a vapor phase.
The constitutive member includes, but is not limited to, a tray supporting member, a packing supporting member, a flange, a nozzle, an end plate (e.g., a flat, dish-form, semi-elliptic, semi-spherical, or conical-form end plate), a column wall, a chimney, a downcomer, a baffle, and a shaft of an agitator. The easily polymerizable compound may be, preferably for example, (meth)acrylic acid, (meth)acrylic esters (methyl(meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, n-propyl (meth)acrylate, isobutyl (meth)acrylate, n-butyl (meth)acrylate, 2-ethylhexylacrylate, 2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and dialkylaminoethyl (meth)acrylates).